In a number of applications, such as the aerospace and automobile industries, structural parts or structural panels are subjected to large loads which produce relatively large stresses and forces on the panels. For example, the upper wing sections of an aircraft are typically compression loaded and may be subjected to crippling and buckling loads. Accordingly, the materials which form the structural parts or panels as well as the resulting structural part or panel must have sufficient strength and stiffness to withstand the anticipated loads.
Traditionally, solid structures have been fabricated which provide sufficient strength. However, solid parts and built-up structures are generally relatively heavy which may limit their utility, such as by undesirably increasing the weight of the resulting aircraft. Thus, materials which can be superplastically formed and diffusion bonded are preferred over materials which are utilized primarily in the production of solid structures.
Superplasticity is the characteristic demonstrated by certain metals and alloys to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metals and metal alloys, has been known in the art as applied to the production of metallic structures as described in detail below. It is further known that at the same superplastic forming temperatures, the metals and metal alloys which exhibit superplasticity can be diffusion bonded with the application of sufficient pressure at contacting surfaces.
Various types of metallic structures have been superplastically formed and diffusion bonded. For example, in a process typically referred to as a three-sheet process, a metallic structure is formed from three metallic sheets, typically an inner sheet and a pair of opposed face sheets. For example, the three-sheet process and the resulting metallic structure is disclosed by U.S. Pat. No. 3,924,793 which issued Dec. 9, 1975 to Summers et al. and is assigned to British Aircraft Corporation Limited; and U.S. Pat. No. 3,927,817 which issued Dec. 23, 1975 to Hamilton, et al. and is assigned to Rockwell International Corporation (hereinafter the '793 and the '817 patents, respectively).
Both the '793 patent and the '817 patent describe methods for superplastically forming metallic sandwich structures from metallic sheets which are joined at selected areas and expanded superplastically. According to the three-sheet method disclosed by these patents, however, the metallic sheets are generally bonded, such as by diffusion bonding, prior to any superplastic forming operations. In particular, the inner sheet is bonded to the outer face sheets. Thereafter, by applying tensile stress to the face sheets, such as by applying gas pressure between the face sheets, the inner sheet is drawn outwardly with the expanding face sheets to which it is joined during the superplastic forming operation.
The three-sheet process generally produces a metallic sandwich structure which has a truss core structure. The truss core structure produced by the three-sheet process does not typically include significant transverse stiffening, but, instead, includes one or more canted elements which extend between the opposed face sheets at an angle which is not perpendicular to the face sheets. In other words, the angle defined between the canted element and a face sheet is less than 90.degree..
In order to produce structural panels which include, among other things, increased transverse strength or stiffness, another superplastic forming and diffusion bonding process, typically termed a four-sheet process, has been developed. An exemplary four-sheet process is disclosed in U.S. Pat. Nos. 4,217,397 and 4,304,821 (hereinafter the '397 and the '821 patent, respectively), both of which issued to Hayase, et al. and are assigned to McDonnell Douglas Corporation. The respective disclosures of both the '397 and '821 patents are hereby incorporated by reference. In general, the '397 patent discloses a four-sheet metallic sandwich structure having a pair of core sheets and a pair of opposed face sheets, while the '821 patent discloses the corresponding method of fabricating the structure disclosed in the '397 patent.
More specifically, the '397 patent discloses a metallic sandwich structure in which metallic core sheets are joined by an intermittent weld. The joined core sheets are thereafter sealed by a continuous weld, such as along corresponding edge portions, to form an expandable envelope. Following the placement of the joined core sheets in a limiting structure, such as a containment die, an inert gas is injected to interior portions of the joined core sheets to thereby superplastically form or expand the core sheets. In particular, by applying appropriate pressure and temperature to the assembled structure, the core sheets are expanded against and diffusion bonded to the surrounding face sheets, thereby producing the resulting structural panel. The core configuration of the resulting structural panel is generally defined by the intermittent weld pattern, as described in detail in the '397 and '821 patents.
Regardless of the type of superplastic forming and diffusion bonding process, both the core sheets and the face sheets are generally formed of titanium alloys, such as Ti--6Al--4V, that are relatively ductile and have a refined equiaxed microstructure and, are therefore, formable during a superplastic forming process. As known to those skilled in the art, Ti--6Al--4V contains 6% aluminum by weight and 4% vanadium by weight. Other common titanium alloys for superplastic forming applications are Ti--6Al--2Cr--2Mo--2Sn--2Zr, Ti--4Al--4Mo--2Sn--0.5Si and Ti--6Al--6V--2S. Although conventional alloys are sufficiently ductile for superplastic forming operations, an increasing number of applications are demanding components, such as structural panels, having increased strength and/or stiffness while weighing the same or less than the same components fabricated from these conventional titanium alloys. For example, the face sheets of an aircraft wing must withstand the majority of the loading forces. As a result, the stiffness or modulus of elasticity (hereinafter "modulus") of the face sheets is extremely important in any attempt to increase the stiffness of the aircraft wing while decreasing or at least not increasing the weight of the aircraft wing.
Therefore, while ductility remains an important material property for components that are superplastically formed, the modulus of elasticity is also becoming of increasing importance in a number of applications that require components having increased stiffness without a corresponding increase in weight. In this regard, Ti--6Al--4V has a modulus of about 16.5.times.10.sup.6 lb/in.sup.2. Of the conventional titanium alloys, Ti--62S has the highest modulus of about 17.3.times.10.sup.6 lb/in.sup.2. While the modulus of these conventional alloys is sufficient for many applications, an increasing number of applications are demanding stiffer materials, i.e., materials having an even greater modulus. In addition, the processes for forming sheets of monolithic titanian alloys require relatively close control of the temperature to maximize formability and to minimize undesirable coarsening. Coarsening decreases further formability and also causes the as-formed properties to be weaker and less ductile. Generally, a temperature excursion of more than 30.degree. F. above the optimal superplastic forming temperature will result in coarsening.